Conventionally, in storage systems, when a storage apparatus receives a request to write data to a disk, the storage apparatus first writes the data to memory prior to writing the data to the disk and thereafter, responds to the writing request, whereby improved response performance is facilitated. Thus, actual writing to the disk is performed after writing to memory.
On the other hand, until writing to the disk is performed, the data is only on the memory and during this time, if an abnormal event occurs at the memory, the data is at risk of being lost. Therefore, a backup apparatus that duplicates and manages the data that is only on the memory, is separately prepared.
For example, the storage apparatus performs data backup by transmitting to the backup apparatus, a backup of the data held in the memory. Since available areas will become insufficient if the backup data continues to accumulate in the memory, the backup apparatus, in response to a backup data deletion request from the storage apparatus, releases storage areas to which the backup data is written.
A related conventional technology has duplicate controllers performing writing to a disk and the respective caches of the controllers are mutually duplicated (see, for example, Japanese Laid-Open Patent Publication No. H9-146842). Further, a technology, when cache has been duplicated, writes data to one cache and responds to a writing request; and thereafter, writes the data to another cache (see, for example, Japanese Laid-Open Patent Publication No. 2005-44010).
However, with the technology above, a problem arises in that increases in the volume of communication between the storage apparatus and the backup apparatus are invited since requests to delete backup data are transmitted from the storage apparatus to the backup apparatus.